Method of manufacturing a bistable magnetic actuator

ABSTRACT

A magnetic actuator (10) suitable for the operation of electric circuit breakers which uses a laminated yoke structure (12) to increase permanent magnet flux holding forces. The actuator comprises a magnetic yoke (12) which forms both low and high reluctance flux paths with at least one permanent magnet (30) and an armature (40) axially reciprocable in a first direction within the yoke (12). The actuator is configured to provide a first low reluctance flux path and a first high reluctance flux path when the armature (40) is in a first position and a second low reluctance flux path and a second high reluctance flux path when the armature (40) is in a second position. A pair of electromagnetic coils (60, 61) are used to drive the armature (40) between the first and second positions. The geometric design of the actuator is such that by increasing one linear dimension of the device by adding lamination to the yoke and making corresponding increases in the same linear dimension of magnet and armature the permanent magnet flux can be increased to meet any specification of device required using the same basic components. The design of the laminated yoke is adapted to considerably improve the low reluctance path to form a more compact device and provide higher holding forces and faster switching times.

The present invention relates to magnetic actuators, and in particularto actuators suitable for the operation of electric circuit breakers.

In all electric circuit breakers it is necessary to have a mechanismthat will open and close contacts in order to interrupt or close anelectric circuit.

Conventional high-voltage circuit breakers include mechanical systemsfor opening and closing the circuit breaker contacts that are verycomplex to build and require periodic and expensive overhaul andmaintenance. The advent of modern vacuum interrupters for use in highvoltage circuit breakers, requiring no maintenance or overhaul, has ledto the desire to make available actuator mechanisms requiring little orno maintenance and ideally matched to the characteristics of the vacuuminterrupter.

These characteristics typically include: short stroke of the movingcontact between open and closed positions, usually of the order of 8 to12 mm; low operating times, typically 10 milliseconds between open andclosed positions during operation; high pressure force between contactswhen closed to withstand electromagnetic forces during short circuits;and low operating energy.

Prior art bistable permanent magnet actuators meet some of the abovecharacteristics but typically have a number of disadvantageous features.

For example, in UK Patent Application No. 2112212 there is described arelay which has a bistable permanent magnet actuator. This relayincludes an electromagnetic coil wound around the armature to providethe necessary electromagnetic driving force to move the actuator betweenthe two bistable positions. This design has a number of disadvantages,not least that the flux generated by the coil works in opposition to thepermanent magnet flux, thus having a tendency to destroy the permanentmagnets in time. Additionally, considerable flux must be generated tooppose and overcome the permanent magnet flux, and the movement of theactuator is thus rapid and substantially uncontrolled. These types ofdevice are inherently unsuitable for actuators requiring large holdingforces, as they will suffer considerable damage when electromagneticfluxes large enough to overcome the permanent magnet flux are generated.They thus have application only in lower power roles. In addition, thecoil is mounted on the moving part (the actuator) thereby requiring amore complex and less reliable configuration.

In a further example, UK Patent Application No. 2223357 there isdescribed a bistable, magnetically actuated circuit breaker. This deviceincludes a dual yoke construction, each yoke providing either the lowreluctance permanent magnet flux path or the high reluctance path of thebistable configuration. The permanent magnet is housed between twohalves of the actuator. Actuation is provided by one of twoelectromagnetic coils which operate to destabilise the armature withoutsubstantially reducing the flux in the permanent magnet. A substantialdisadvantage of this device is that the magnet is located in thearmature, and thus for actuators requiring large holding forces, isprone to physical damage under the impact of switching the armatureposition. A further substantial disadvantage of this device is that theconduction of permanent magnet flux around the device is inefficient andlarge magnets are required to achieve reasonable holding force.Similarly, generation of electromagnetic flux is inefficient and largeswitching currents are required.

Where prior art designs of actuator have been made to accommodate highpower circuit breakers requiring large holding forces, it has alwaysbeen necessary to provide electromagnetic coils capable of generatingvery large opposing fluxes in order to switch the actuator from onebistable position to the other. While this is not always a problem, itis particularly difficult where the breakers must have an independentsource of power in order to switch, such as those which must be poweredby integral batteries which are required to have a long,maintenance-free life. In addition, the use of high power coilsnecessarily increases the size of the actuators, and may necessitateexpensive cooling mechanisms where frequent switching occurs.

There is therefore a need to provide a permanent magnet actuator whichis simple and cheap to manufacture, suitable for use with high powerapplications generating large holding forces, with substantially lowerpower consumption than known systems, and easily configurable to avariety of specifications.

In accordance with one aspect of the present invention, there isprovided a bistable permanent magnet actuator comprising:

a magnetic yoke;

at least one permanent magnet; and

an armature axially reciprocable in a first direction within the yoke;the actuator configured to provide:

a first low reluctance flux path and a first high reluctance flux pathwhen the armature is in a first position;

a second low reluctance flux path and a second high reluctance flux pathwhen the armature is in a second position;

means to drive the armature between the first and second positions;

characterized in that:

the yoke comprises a laminated structure.

In accordance with a further aspect of the present invention, there isprovided a method of manufacturing a bistable permanent magnet actuatorcomprising the steps of:

constructing a magnetic yoke from a plurality of laminations eachconfigured to form a part of a magnetic circuit with at least onepermanent magnet and an armature axially reciprocable in a firstdirection within the yoke;

configuring the actuator to provide a first low reluctance flux path anda first high reluctance flux path when the armature is in a firstposition and a second low reluctance flux path and a second highreluctance flux path when the armature is in a second position;

providing means to drive the armature between the first and secondpositions; and

using a predetermined number of laminations to expand the device in alinear direction orthogonal to the plane of the yoke laminations, andincreasing the corresponding linear dimension of the magnet(s) andarmature in order to increase in the permanent magnet flux flowingthrough the actuator to achieve the desired specification of actuator.

Embodiments of the present invention will now be described by way ofexample, and with reference to the accompanying drawings in which:

FIG. 1 shows a perspective view of part of a magnetic actuator inaccordance with one embodiment of the present invention, with one coiland yoke laminations removed to reveal internal components;

FIG. 2 shows an end view of a centre cross-section of the completeactuator of FIG. 1;

FIG. 3 shows a side view on cross-section A--A of the actuator of FIG.2, but with the leading part of both coils removed for clarity;

FIG. 4 shows a top view on cross-section B--B of the actuator of FIG. 2,but with the upper coil removed for clarity.

With reference to the figures, a bistable, permanent magnet actuator isshown generally as 10. The actuator comprises an outer yoke 12, which iscomposed of a number of laminations 14,15 formed of a suitably highmagnetic permeability material, for example steel sheets. Eachlamination has an upper and a lower pole portion 16,17 and preferablyincludes a pair of centre arms 19,20 projecting inwards from sideportions 22,23. Although the preferred embodiment has been shown assymmetrical about a vertical centre line on FIG. 2, it will beunderstood that one of the side portions 22,23 could be omitted.

Within the laminations 14,15 of yoke 12, and preferably lying betweenand adjacent to centre arms 19,20 are a number of permanent magnets 30.Magnets 30 are attached to a pair of inner yokes 31,32 which are spacedfrom an armature 40 which is reciprocally mounted within the assembly inorder that it may slide between a first, lower position in which thelower face of the armature 30 is in contact with the lower pole portion17 of yoke 12 as shown in FIG. 2, and a second upper position in whichthe armature is in contact with the upper pole portion 16 of yoke 12.Coaxial with the armature 40 is an actuator rod 42 shown in dottedoutline on the figures. Four bearing plates 50 . . . 53 (see FIGS. 3 and4) are positioned between the ends of inner yokes 31,32 and the armature40 to facilitate smooth linear movement of the armature within theyokes.

A pair of coils 60,61 circumscribe the upper and lower portions ofarmature 40 respectively. The coils are preferably mounted within therecesses formed between the poles 16,17 of the yoke 12 and the centrearms 19,20. The whole assembly may then be bolted together and providedwith end caps 70,71.

With the armature 40 in the position as shown in the figures, a lowreluctance magnetic circuit is formed by the magnet 30, the lower halfof side portion 22 of yoke 12, the lower pole 17 of yoke 12, the lowerhalf of armature 40 and the inner yoke 32. A high reluctance magneticcircuit is formed by magnet 30, the upper half of side portion 22 ofyoke 12, the upper pole 16 of yoke 12, the upper half of armature 40 andthe inner yoke 32. Corresponding circuits are replicated on the lefthalf of the actuator as viewed in FIG. 2.

In this position, a strong permanent magnet flux is present in the lowreluctance circuit which holds the armature in the lower position.Little flux is present in the high reluctance circuit due to the air gap62 present between the upper part of the armature 40 and the upper pole16 of the yoke 12. However, it will be recognized that the temporaryapplication of a current of appropriate polarity in upper coil 60 willcause a high flux to be forced across the air gap 62, providing anupward motive force on armature 40 in order to close the air gap.Providing the flux induced by coil 60 is greater than the flux presentin the low reluctance circuit, the armature will be "flipped" to anupper position; thus swapping over the high and low reluctance circuitsdescribed supra.

The armature may be returned to its first bistable position by analogoususe of the lower coil 61.

This action offers considerable improvement over some types of actuatorin that the coils never serve to oppose the permanent magnet flux, andthus do not tend to destroy the permanent magnets over time.

The use of an outer yoke 12 comprised of a number of laminations hasseveral important advantages. Firstly, the permanent magnet flux flowingthrough the low reluctance circuits is greatly improved for given magnetstrengths: this enables a very substantial increase in the holding forceof the actuator for a given magnet strength and for a given size ofactuator. Additionally, the transient power consumed by coils 60,61 toswitch the armature from one bistable position to the other issubstantially reduced as more efficient flux generation in the yoketakes place. Not only does this result in a substantially reducedcurrent consumption during switching, but it is discovered thatsubstantially shorter current pulse times can be used to effect theswitching operation.

Improvements in the performance of the device are also found with theuse of the "one-piece" outer yoke lamination configuration: that is tosay, both the low reluctance path and the high reluctance path of abistable position are provided in the same structure (ie. in eachlamination). This also assists in the transient flux generation by theappropriate coil 60,61.

Traditionally, prior art devices have been constructed around acylindrical armature with a cylindrical yoke, or separate yokes radiallyspaced around the outside of the cylindrical armature. A substantialadvantage in the particular geometrical configuration of actuatorillustrated in the figures is that devices of varying specification canbe manufactured using standard parts. By increasing the number oflaminations 14,15 used, the number of magnets 30 used, and the length ofarmature, the device is expandable along the axis perpendicular to theplane of the laminations. This permits any desired size of device to bemanufactured, and increasing length provides greater and greater holdingforce of the finished actuator. Thus, actuators can readily bemanufactured to provide just sufficient holding force for any particularapplication, while avoiding the necessity of using substantiallyover-specified devices which use more current than strictly necessaryfor the application. It will be understood that in similar manner to thelamination of the yoke, the armature 40 could also be laminated insimilar manner for optimum versatility.

In practice, it is not essential to use an inner yoke 31,32 providingthat some means to attach the magnets to the outer yoke is provided.

An additional preferred feature is the provision of the armature in twohalves 40a, 40b as shown in FIG. 2. This considerably eases the assemblyof the actuator. When constructing an actuator, very considerable forcesmust be overcome to place magnets and armature in position to completethe magnetic circuits. It is preferable to assemble the actuator withunmagnetised "permanent magnets". The two armature halves have a "slug"of high permeability material introduced between them and are then slidinto position between the respective upper and lower pole portions 16,17of the outer yoke 12. The slug effectively expands the armaturesufficiently so that the air gap 62 is eliminated. The remaining partsof the actuator are assembled, with the exception of actuator rod 42.Magnetisation of the magnets 30 then takes place by energising bothcoils in such a way that the desired polarity of magnets 30 are created.

The slug is then removed, and the actuator rod 42 is passed through theupper pole portion 16 of the yoke and into a preformed hole in the upperhalf of the armature. The lower end of the actuator rod 42 is threaded,as is the corresponding preformed hole in the lower half of thearmature. The two halves of the armature may thus be brought together byscrew threading the actuator rod into the hole in the lower half of thearmature. Thus, the necessary mechanical advantage to overcome themagnetic forces is provided by suitable torque on the actuator rod 42.

We claim:
 1. A method of manufacturing a bistable permanent magnetactuator comprising the steps of:constructing a magnetic yoke from aplurality of laminations each configured to form a part of a magneticcircuit with at least one permanent magnet and an armature axiallyreciprocable in a first direction within the yoke; forming the armaturein two halves by division of the armature by a plane orthogonal to saidfirst direction; introducing a slug of high permeability materialbetween the two halves of the armature and installing the armature andslug into the yoke; removing the slug and installing an actuator rodadapted to draw together said two armature halves in a directionparallel to said first direction; configuring the actuator to provide afirst low reluctance flux path and a first high reluctance path when thearmature is in a first position and a second low reluctance flux pathand a second high reluctance flux path when the armature is in a secondposition; providing means to drive the armature between the first andsecond positions; and using a predetermined number of laminations toexpand the device in a linear direction orthogonal to the plane of theyoke laminations, and increasing the corresponding linear dimension ofthe at least one magnet and armature in order to increase in thepermanent magnet flux flowing through the actuator to achieve thedesired specification of actuator.
 2. A method of manufacturing abistable permanent magnet actuator according to claim 1, furthercomprising the steps of:installing the at least one permanent magnet inan unmagnetised state; after installation of the armature and slug, andbefore removal of the slug, magnetizing the at least one permanentmagnet in situ.